The present invention relates to a system for moving a movable members and more particularly to an improved ballnut mounting arrangement in a power assisted steering system.
Power assisted steering systems assist a vehicle driver in turning the steerable wheels of a vehicle. In a rack and pinion steering system, the driver applies rotary force to a steering wheel of the vehicle and this rotary force is transferred through the steering input shaft to a pinion that interacts with a rack to cause the steerable wheels to turn. A power assisted steering system assists the driver by either applying a rotary force to the steering input shaft to assist in driving the pinion or by applying a linear force directly to the rack.
When applying a linear force directly to the rack, a power assisted steering system generally uses a hydraulic or an electric motor. When an electric motor is used, the electric motor may be connected to a ballnut assembly that converts the rotary force of the electric motor into a linear force to move the rack. The electric motor drives a ballnut assembly that encircles a threaded portion of a steering member. When the electric motor is energized, the rotary motion of the motor rotates the ballnut about the steering member causing the steering member to move linearly.
The ballnut may be mounted within the cylindrical motor shaft of the electric motor. It is common to fix the ballnut both radially and axially relative to the motor shaft of the electric motor. When the ballnut is both radially and axially fixed, assembly of the power assisted steering system requires close manufacturing tolerances to insure proper alignment of parts. Additionally, during use, the environmental factors acting upon the steering member of the power assisted steering system vary continuously. Examples of such environmental factors are road conditions, vehicle load, vehicle speed, steering rate, temperature, and steering direction. As a result of these factors, the steering member may bend slightly and may apply varying axial and radial forces against the ballnut. These forces may cause increased friction between the ballnut and the steering member resulting in noise and in loss of efficiency of the power assisted steering system.
The present invention is directed to a system which includes a rotatable member capable of rotating about an axis. The system includes a ballnut coaxial with the rotatable member. The ballnut has a threaded cylindrical inner surface and a plurality of drive balls. A movable member is coaxial with the rotatable member and the ballnut. The movable member has a threaded portion. The movable member extends through the inner surface of the ballnut such that the drive balls of the ballnut engage the threaded portion of the movable member. The ballnut is rotatably driven about the axis by the rotatable member and is pivotal relative to the rotatable member in a direction transverse to the axis.
In one embodiment of the invention, the ballnut is pivotal upon two pivot balls, located on opposite sides of the axis. Each pivot ball is located in a recess formed in the outer surface of the ballnut and in an opening formed in the rotatable member.